As C-3PO might say: R2-D2, what are you? What are you?

A mystery seller just auctioned off a version of the famous Star Wars droid -- one that was assembled, Frankenstein's monster-like, from parts used in separate Artoos in five separate George Lucas-made movies -- to an anonymous buyer for $2.76 million.

That's $1.6 million more than the starting price, and $760,000 more than the expected maximum.

But questions about the legitimacy of this particular droid have been raised by members of the international R2 Builders Club, the organization of droid lovers that Lucasfilm worked with to construct the Artoo unit seen in The Force Awakens. Several members claim that its dome matched the description of one that went missing from the set of Star Wars Episode I.

The website that hosted the auction, Invaluable.com, and the California-based seller Profiles in History, were unable to answer questions from Mashableabout the origins of the droid, citing the seller's privacy. The auction description says this Artoo is currently in the UK, where all Star Wars films including Episode Iwere made.

"This R2-D2 offering represents the pinnacle of the Star Warscollecting universe," reads the mystery seller's description. It "was put together over many years by sourcing original components and assembling them as a complete R2-D2 ... given the ad hoc nature of production practices, any 'complete' existing R2-D2 units from the first trilogy (in studio hands) would be a compilation very similar to this R2-D2 unit offered here."

Multiple sources within the R2 Builders Club confirmed that the parts seemed authentic, but also said they recalled the dome being offered for sale on the Yahoo forum the club formerly used. When the seller was told that a well-known Lucasfilm veteran was a member of the forum, they disappeared, the builders said.

The underworld of Star Wars prop collectors is not always the most salubrious place, as we learned earlier this month when the owner of the world's largest Star Wars collection shared the heartbreaking tale of a former friend and R2 Builder's Club member who had stolen items worth $200,000. (That owner, Steve Sansweet, confirmed to Mashablethat the mystery Artoo unit was not connected to his collection and that he would not be bidding.)

There were other items in the Profiles in History auction -- most notably the "hero" lightsaber used by Luke Skywalker in both Star Warsand Empire Strikes Back(in fictional terms, this is the same lightsaber Rey hands back to Luke at the end of The Force Awakens). The lightsaber sold for $450,000, some $200,000 more than expected.

The original owner of that particular prop was not anonymous, however. It was Gary Kurtz, producer of the first two Star Wars movies, who provided a letter of authenticity.